1. Field of the Invention
The present invention relates generally to an apparatus and a process for the removal of insoluble substances from a liquid. More particularly, the present invention relates to a flotation apparatus and process for removing oils, solids, and other insoluble and/or suspended substances from produced water which is typically generated during the production of crude petroleum and natural gas.
2. Description of Related Art
Water contaminated with small amounts of oil, suspended solids, and/or other insoluble substances is a byproduct of many industrial operations such as, for example, the production and refining of crude petroleum and natural gas; the manufacturing of chemicals, paint, textiles, paper and pulp; the generation of electricity; and the processing of metals, food and beverages. A major problem faced in these industrial operations is the economic separation of the oil, suspended solids, and/or other insoluble substances from the water so that the water can be reused or discharged into the environment.
This separation problem is especially prevalent in the offshore production of crude petroleum and natural gas, where many oil and gas wells produce a certain amount of water or brine (xe2x80x9cproduced waterxe2x80x9d) in conjunction with the oil and gas. The produced water is typically ladened with oil and other dissolved contaminants, and before it can be reinjected downhole or disposed back into the sea, the oil and other dissolved contaminants must be removed. Accordingly, numerous methods and systems have been devised to reduce the content of oil and other dissolved contaminants from produced water derived from offshore oil and gas wells.
One such method and system specially adapted for offshore operations is the skim pile, which is essentially a settling vessel consisting of a long vertical pipe that is installed adjacent to the offshore production platform. The skim pile normally extends from the lower deck of the platform to near the sea floor. Produced water containing oil and other dissolved contaminants is continuously introduced near the lower end of the skim pile and is permitted to separate into two layers, one layer being a relatively thin film of oil and other contaminants which forms at the top of the skim pile, and the other layer being a clarified zone of produced water which forms at the bottom of the skim pile. The oil and other contaminants are withdrawn from the top of the skim pile while the clean water is allowed to flow out the open bottom.
Two major limitations of skim piles are that they can only handle relatively low flow rates and they are generally ineffective at removing finely suspended solids or insoluble liquid droplets. Furthermore, it is difficult, if not impossible, to monitor the amount of oil and other contaminants flowing out the open bottom into the sea. For these and other reasons, skim piles are no longer the preferred method for separating oil from produced water. In fact, skim piles have been banned in the United States because of environmental concerns.
Other types of separation methods and systems, such as coalescers, have been developed and implemented to replace skim piles. Coalescers utilize a filter element to absorb and coalesce oil particles and other suspended contaminants from the liquid. (See U.S. Pat. No. 3,794,583.) A drawback is that the filter elements typically have a complex internal design and tend to get saturated with oil and plugged with suspended contaminants, necessitating frequent replacement or cleansing of the filter. Dismantling the unit for clean-up is time consuming and requires the availability of a back-up unit to maintain continuous production from the oil and gas fields. Moreover, the high cost of purchasing and maintaining filtration equipment militates against the use of these techniques in most industrial operations.
Because of the inherent problems associated with coalescers, other types of separation methods, such as gas flotation units, have been employed in the production of crude petroleum and natural gas. Gas flotation units are one of the most versatile methods for the clarification of produced water. These units utilize gas bubbles dispersed throughout the stream of produced water to agglomerate the oils and other contaminants and float them to the surface of the produced water where the gas is released and the oils and other contaminants are collected and separated from the produced water.
Gas flotation devices are well known in the art. A representative but not exhaustive list of the prior art includes U.S. Pat. Nos. 3,452,869; 3,525,437; 3,576,738; 3,616,919; 3,725,264; 3,769,207; 3,784,468; 3,849,311; 3,853,753; 3,884,803; 3,932,282; 4,022,695; 4,086,160; 4,110,210; 4,198,300; 4,255,262; 4,226,706; 4,251,361 ;4,399,028; 4,564,457; 4,627,922; 4,752,399; 4,800,025; 4,889,638; 4,902,429; 4,986,903; 5,080,802; 5,462,669; 5,543,043; 5,580,463, 5,584,995, and 5,693,222.
One specific type of gas flotation process is known as the dispersed-gas method. In a dispersed-gas flotation system, gas in the form of tiny bubbles is mechanically dispersed into the produced water either by means of an inductor device or by a vortex set up by mechanical rotors. The oil is floated to the surface by means of the tiny gas bubbles and thereafter is mechanically separated from the water, thereby producing an effluent water having a substantially reduced oil content. One such dispersed-gas unit is disclosed in U.S. Pat. No. 4,564,457. Another such dispersed-gas unit is disclosed in U.S. Pat. No. 4,255,262.
Another specific type of gas flotation process is known as the dissolved-gas method. In a dissolved-gas flotation system, gas at high pressure is dissolved into a slipstream of the produced water and introduced into a flotation tank at a lower pressure. Microbubbles of gas are released from the slipstream and allowed to rise gently through the produced water inside the flotation tank thereby lifting the oil and other contaminants to the surface.
The micro-bubbles of gas provide an expanded surface area which dramatically increases the probability of gas bubbles coming into contact with the oil and other contaminants contained in the produced water. For instance, a 9 millimeter (9,000 micron) gas bubble would present a surface area substantially smaller than 3 millimeters (3,000 microns). If the bubble size is further reduced to 0.01 to 0.04 millimeters (10 to 40 microns), the surface area yield would be many times greater for the identical gas inlet volume.
A dissolved-gas flotation system is disclosed in U.S. Pat. Nos. 3,576,738 and 3,725,264. Other dissolved-gas flotation systems are disclosed in U.S. Pat. Nos. 3,452,869 and 4,022,695.
There are several disadvantages inherent in both the dispersed-gas and dissolved-gas flotation systems. The first disadvantage is that both systems typically require a relatively large amount of space, creating problems in space critical applications, such as, for example, offshore operations. Dispersed-gas systems, in particular, usually require large moving parts that are high maintenance and energy intensive. For example, dispersed-gas systems usually employ motorized impellers, which are plagued with a number of fabrication, operational and maintenance disadvantages. Likewise, dissolved-gas systems require carefully controlled conditions in order to be effective and further require complex auxiliary equipment, such as pressurization chambers and compression pumps.
Thus, there is a pressing need for a compact flotation system which uses system gas to efficiently and effectively separate oil and/or suspended solids from produced water at high rates to enable reuse of the produced water or to allow discharge of the produced water into the environment. The present invention provides such an apparatus and method.
It is therefore an object of the present invention to provide a gas flotation apparatus and process for the separation of oils, solids, and other insoluble and/or suspended substances from produced water wherein the gas relied upon to facilitate the separation of the oils, solids, and other insoluble and/or suspended substances from the produced water can be the natural gas readily available on most offshore oil and gas production platforms.
It is another object of the present invention to provide a gas flotation apparatus and process for the separation of oils, solids, and other insoluble and/or suspended substances from produced water while occupying a minimum of space in space critical applications.
It is a further object of the present invention to provide a gas flotation apparatus and process for the separation of oils, solids, and other insoluble and/or suspended substances from produced water to generate a purer treated liquid product which can be reinjected down hole or discharged directly into the environment, if required, and an oil product which has significantly less water than the oily product produced by other flotation methods.
It is another object of the present invention to provide a gas flotation apparatus and process for the separation of oils, solids, and other insoluble and/or suspended substances from produced water which apparatus and process do not rely on high shear pumps, mixers, or gas saturated liquids to produce gas bubbles, and which apparatus and process reduce the number of components required for the overall flotation operation.
It is still another object of the present invention to provide a gas flotation apparatus and process which employs a centrifugal pump of the type disclosed in U.S. Pat. No. 5,591,001 to disperse gas bubbles into the produced water.
It is yet another object of the present invention to provide a gas flotation apparatus and process for the separation of oils, solids, and other insoluble and/or suspended substances from produced water wherein contact times are adequate for the gas to rise to the surface and disengage from the produced water.
It is a further object of the present invention to provide a gas flotation apparatus and process which obtains more complete and faster separation of oils, solids, and other insoluble and/or suspended substances from produced water without turbulence and back-mixing.
Another object of the present invention is to provide a gas flotation apparatus and process which can separate dilute concentrations of oils, solids, and other insoluble and/or suspended substances from produced water.
A further object of the present invention is to provide a gas flotation apparatus and process which can separate oils, solids, and other insoluble and/or suspended substances from produced water with the addition of little or no chemicals.
It is a further object of the present invention to provide a gas flotation apparatus and process which have improved means for intimate mixing of the produced water and gas.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art when the following description of the preferred embodiments is read in conjunction with the accompanying drawings.
The present invention solves the problems unresolved by the prior art by using system gas to provide a compact apparatus for effectively and efficiently separating oils, solids, and other insoluble and/or suspended substances from produced water so as to enable reuse of the clarified produced water or to allow discharge of the clarified produced water into the environment.
Briefly, the present invention includes a vessel for receiving and maintaining a liquid level therein. Several process lines allow for the flow of materials in and out of the vessel, including a produced water inlet line, which supplies the vessel with produced water having oils, solids, and other insoluble and/or suspended substances (generally referred to as xe2x80x9coils and other contaminantsxe2x80x9d), and a gas supply line, which supplies a source of gas into the vapor space of the vessel.
A circulating pump provides a means for introducing tiny flotation gas bubbles into the vessel, and is preferably the type described in U.S. Pat. No. 5,591,001, issued on Jan. 7, 1997, to Robert B. Ray, et al. During operation of the pump, liquid is drawn into the pump and mixed with gas received from the vapor space of the vessel. The liquid and gas are mixed in the chamber of the pump and then discharged from the pump, whereupon the gas dissolves into the liquid after being subjected to the discharge pressure of the pump.
The liquid containing the dissolved gas is pumped into the vessel, where the liquid is uniformly distributed over a horizontal plane area preferably below the liquid level contained within the vessel. As the liquid is dispersed into the vessel, the dissolved gas effervescently releases from the liquid, thereby forming tiny, finely dispersed gas bubbles.
The tiny gas bubbles rise vertically through the produced water inside the vessel and attach themselves to the oils and other contaminants suspended in the liquid. Once attached, the gas bubbles reduce the density of the oils and other contaminants causing them to be buoyantly transported to the top of the liquid level in the vessel, where the gas bubbles xe2x80x9cpop,xe2x80x9d thereby releasing the oils and other contaminants. The gas returns to the vapor space of the vessel where it is either recycled back to the recirculating pump or transferred out of the vessel for further processing. The released oils and other contaminants form a film on top of the liquid level inside the vessel and eventually overflow into a reservoir from where they can be recovered from the vessel.
The clarified produced water is removed from the vessel through a liquid outlet line, which is preferably located near the bottom of the vessel. At this point, the clarified produced water is pure enough to reuse in the well or to return to the environment.
Thus, the present invention provides a compact gas flotation apparatus and process which uses system gas to effectively and efficiently separate oils, solids, and other insoluble and/or suspended substances from produced water, as illustrated and described in more detail in the following drawings and description.